1. Field of the Invention
The present invention relates to a process for manufacturing a reinforcing material for concrete. The reinforcing material is of the type which incorporates a multitude of relatively short and substantially straight steel reinforcing elements with substantially rectangular cross section and small thickness in relation to their length and width. These reinforcing elements are intended to be uniformly distributed and in principle randomly orientated in the concrete.
In the present context "concrete" means primarily cement concrete, i.e. a hardened mixture of cement, sand and water, with or without additives such as stone materials, but also other types of concrete such as asphalt concrete. But in certain cases the term "concrete" is also used to define in a general way the unhardened mixture. Further, "steel" means a material with the element iron as its principle component, produced by a smelting process or other suitable method, e.g. by direct reduction, and with a carbon content of between 0% and about 2.0%, preferably not exceeding about 0.3%. It must always, however, be ensured that the reinforcing elements are not brittle and easily crushed, but possess a certain flexibility and ductility so that they stand being mixed with unhardened concrete without being crushed. As a rule, the hardness of the flakes should be at most about HRC 50. Temperable steels may require dead-soft annealing to give the reinforcing elements the desired properties.
2. Description of the Prior Art
A reinforcing material of said type is described in the British pat. specification No. 303,406 and consists of elongated flat or ribbon-like strips or shreds of plate cut out of flattened tins and similar waste material. This reinforcing material seems however never to have found practical usage, probably due in part to a shortage of suitable raw material (i.e. empty tins and such like) and the high transportation and cleaning costs for the raw material. Due to the plate's susceptibility to corrugate when the tins are flattened and to differences in dimensions and materials between tins, it may also have been impossible to produce a homogeneous reinforcing material in which individual reinforcing elements had mutually identical properties and the same dimensions.
The use of these metallic fibres of circular or rectangular cross section to reinforce synthetic resin materials is known. E.g. the U.S. Pat. No. 3,231,341 describes such fibres having a length of 0.05 to 50 mm and a "mean diameter" in cross section of 0.006 to 0.25 mm. For rectangular cross sections this "mean diameter" is half the sum of the short side and the long side of the rectangle. The fibres are suited for manufacture of metal fibre reinforced sealing members, bearings and similar small components. However, due to their small dimensions they as a rule are too expensive to be used in concrete.
A similar reinforcing material but for concrete is described in the U.S. Pat. Nos. 3,429,094 and 3,500,728 and consists of short and straight steel wires with smooth surfaces and circular cross sections, whose diameters can vary between about 0.15 and 0.60 mm, and whose lengths can vary between about 10 and 75 mm depending on the field of application. Compared to unreinforced concrete, the wire-reinforced concrete exhibits greater flexural strength, compressive strength, impact resistance, abrasion resistance and spalling resistance. The wire-reinforced concrete also exhibits much greater resistance to crack formation and thermal shock, and the sections can be made considerably thinner for a given design strength, which results in material saving. Further, other types of reinforcement can be dispensed with and the labour cost to install reinforcement can be eliminated, and the wire-reinforced concrete requires less maintenance and has a longer service life.
Wire-reinforced concrete has been chiefly used in the construction of airfields and roads, pre-cast units, e.g. for the building industry, in situ or pre-cast tunnel linings, marine applications, etc., but employment on a large scale has been retarded by the cost of producing the thin short wires. The production of thin wire requires a long series of rolling and drawing operations and is consequently expensive. An example can be quoted in illustration; about six die reductions would be needed just to form round wire of a diameter of 0.254 mm to round wire of a diameter of 0.127 mm.
The use of wire-reinforcement in concrete is based on the theory of crack retardation in composite materials of the type "brittle material/ductile fibre," see e.g. The Journal of the Australian Institute of Metals, 16 (171):4, p 204-216 (W. J. McG. Tegart, Principles of Composites), and Metal Science Journal, 3 (1969), p 45-47 (S. D. Antolovich, Fracture Characteristics of a Brittle-Matrix/Ductile-Fibre Composite). When a crack occurs it is retarded by the fibres lying superficially. If the crack deepens, the fibres must accept the load and stretch elastically. If the stress on a fibre becomes too great, the fibre breaks and crack propagation proceeds. Alternatively the stress can exceed the adhesion between concrete and fibre, whereupon slippage starts, the fibre cannot be exploited and the process continues. In a mixture of concrete and randomly orientated steel fibres only a small proportion of the fibres are effective regarding crack retardation, namely those which are orientated parallel or nearly parallel to the direction of the tensile stress, since in this context a wire has only one effective direction, namely its longitudinal direction.
With regards to the stress distribution in a reinforcing element, when concrete reinforced by a large number of randomly orientated elements of this type is loaded, it can be confirmed that the maximum stress will occur in the middle of a reinforcing element orientated parallel to the direction of stress, whereas the element's ends are unstressed and thus ineffective from the reinforcement aspect. Since a wire as well as a strip or a fibre has a constant cross sectional area throughout its length, no one of them is in this respect an ideal reinforcing element. The volumetric content of reinforcing elements is utilized only to a fraction.
In order to achieve the overall reinforcing effect aimed at, bonding between concrete and reinforcing elements must be good. The loading is transferred from the concrete to a reinforcing element by shearing forces acting throughout the latter's length at the interface between element and concrete. If the reinforcing elements are short straight wires with circular cross sections and smooth surfaces, it is clearly difficult to achieve good bonding between the elements and the concrete. It has therefore been proposed, see the U.S. Pat. No. 3,592,727, to use wire which has a plurality of fairly long, flattened parts of substantially rectangular cross section, joined by shorter parts of substantially circular cross section. Such wire can be made by a special rolling process, and the flattened parts can have a ratio of width to thickness of between 1.5 to 1 and 5 to 1. In the special rolling process a start is made with a wire with a maximum diameter of 0.75 mm, which means that in addition to the production of the modified wire expense is incurred through the long series of drawing operations required just to make a wire with a maximum diameter of 0.75 mm. Neither is any improvement achieved regarding crack retardation, and furthermore the wire's cross sectional area is still constant throughout the length of the wire.
For a composite material comprising concrete and a large number of short, straight steel wires to acquire optimal properties, the wires must be distributed as uniformly as possible in the concrete. Since steel wire has a considerably higher density than concrete, there is a certain risk of settling and even of local stacking, especially when vibrators are used to pack the concrete. The above-mentioned modified wires fail to give any improvement in this respect.